This invention relates to a system for controlling a plurality of vehicle safety devices, and particularly to an improvement of a circuit for checking the operability of actuating elements of the vehicle safety devices.
It is known that a driver's seat and an assistant driver's seat of an automobile vehicle are equipped respectively with air-bags (vehicle safety devices). U.S. Pat. No. 5,155,376 discloses a system for controlling the two air-bags as shown in FIG. 2. In a feeder circuit of this control system, two squibs (actuating elements) of the two air-bags are connected in parallel to a condenser (power source) having a large capacity. The two squibs are connected in series with two transistors (switching means), respectively. The control system further comprises a microcomputer. This microcomputer is operated to judge whether or not there is a vehicle collision in accordance with an acceleration signal coming from an acceleration sensor and output trigger signals respectively to the transistors when it judges that a vehicle collision has occurred. The transistors are turned ON in response to the trigger signals. As a result, electric current from the condenser is supplied to the squibs to ignite the squibs, thereby inflating the air-bags. The reason why the squibs are connected in parallel to each other is that by doing this, sufficient amounts of electric current are supplied respectively to the squibs upon the occurrence of a vehicle collision, so that the two air-bags can be assuredly inflated.
When one or both of the squibs are subjected to a malfunction (for example, short-circuited or accidentally opened), the air-bags become impossible to be inflated. Therefore, it is always necessary to monitor or check as to whether or not each squib is subjected to a malfunction. For this purpose, the control system disclosed in the above U.S. Patent Publication has a single electric current controlling circuit and two detection circuits. This electric current controlling circuit is operated to turn ON the two transistors to flow very weak check current in parallel to the two squibs, respectively. The detection circuits obtain a detection voltage in accordance with a voltage generated between opposite ends of the corresponding squibs, respectively. The microcomputer judges whether or not each detection voltage is within a predetermined allowable range of voltage, and outputs a malfunction detection signal to a display device when the judgement result is "NO".
The above control system requires a plurality of detection circuits corresponding to the number of the squibs, and therefore the cost is increased. In the microcomputer, since the malfunction diagnosis is carried out in accordance with the detection voltages from the squibs, the number of operation times is increased. Instead of the microcomputer, when discreet circuit such as, for example, a window comparator is to be used as malfunction diagnosing means, a plurality of window comparators are required and therefore the cost is increased. Furthermore, since the electric current control circuit is operated to turn ON the transistor to flow a check current to each squib in the state where there is no vehicle collision, there is a possibility that the air-bags are accidentally inflated due to malfunction of the electric current control circuit.
Japanese Laid-Open Patent Application No. 158445/90 discloses a system for controlling a single air-bag. In this control system, a very weak electric current is normally flowed in a feeder circuit, the voltage generated between opposite ends of a squib arranged in the feeder circuit is watched by a window comparator, and when this voltage is brought to be outside the allowable range, the window comparator actuates an alarm lamp.